1. Field of the Invention
The present invention relates to an apparatus and a method of measuring the transmission velocity of a pulse wave. The method and apparatus according to the present invention are used for detecting the transmission velocity of a pulse wave in relation to the detection of the blood pressure and the elasticity of the tube wall of the artery of a human being.
2. Description of the Related Arts
In a report based on an investigation into the relationship between the degree of sclerosis of an artery and the pulse wave velocity (PWV), it was assumed that C.sub.o is the value of the pulse wave velocity, V is the volume of the artery, P is the internal pressure of the artery, .rho. is the density of the blood (regularly 1.055 g/cm.sup.3), and V dP/dV is the bulk modulus (volumetric elasticity), the value of C.sub.o is represented by the formula ##EQU1## As a result it was found that, if the density of the blood is constant, the harder the tube wall of the artery, the higher the pulse wave velocity.
Also, it is known that the pulse wave velocity (PWV) in the aorta can be calculated based on detections of the pulse wave in the carotid artery, the pulse wave in the femoral artery, and the heart sound, using the formula EQU PWV=1.3L/(T+T.sub.c).
In this equation, L represents the straight distance from the valve opening of the aorta to the femoral artery, T the time difference between the rising point of the pulse wave in the carotid artery and the rising point of the pulse wave in the femoral artery, and T.sub.c the time difference between the generation of the second sound, i.e., the sound of the closing of the aortic valve, of the heart sounds to the generation of the dip of the pulse wave, which is generated when the aortic valve is closed, in the carotid artery Accordingly, "T+T.sub.c " is the time of a transmission of the pulse wave from the opening of the aortic valve to the femoral artery. The coefficient "1.3" is the correction coefficient of the actual length of the artery.
Recently, a demand has arisen for a precise measurement of the PWV over a relatively short distance, as the degree of the sclerosis of the tube wall of an artery in a relatively localized range can be detected by such a precise measurement of the PWV over a relatively short distance The detection of the localized existence of the sclerosis in the artery system, in association with the detection of the sclerosis in the entire artery system, is useful for the medical diagnosis and treatment of the vascular diseases accompanying sclerosis of the artery.
Since the carotid artery is located at the entrance of the cerebral blood vessel system, the degree of sclerosis of the tube wall of the carotid artery is considered to be as medically important as the heart artery system.
To achieve a precise measurement of the PWV over a relatively short distance, a measurement with a high time resolution is needed, since a very short transmission time on the order of milli-seconds through tens of milli-seconds of the PWV is estimated as the transmission time of the PWV over a short distance on the order of 5 cm.
In a prior art method of measuring the PWV, in which the PWV is obtained from a measurement of the time difference between 1/10 points or 1/5 points of the amplitudes of the rising parts of two pulse waves having a relatively stable pulse waveform, a problem arises in that the time difference is measured for only a single point, and if noise components are superposed on the pulse wave signal, tho amount of error in the time measurement is increased. Such an error cannot be neglected, particularly in the measurement of the PWV over a short distance on the order of 5 cm.
In another prior art method of measuring the PWV, in which the waveforms of the rising parts of two pulse waves are overlapped by using an analog delay element and the delay time therebetween is measured, a problem arises in that the process of the decision based on the overlap of the waveforms is carried out only by a visual fine adjustment of an oscilloscope by the operator, an automatic measurement of PWV for each heart beat cannot be made, only an averaged value is measured because the pulse wave signals for several heart beats are required for the overlapping of the waveforms, and the dynamic response characteristic for a load test, such as the test of an increase in pressure under cold temperature conditions, cannot be detected.
To obtain information on the background of the invention, refer to an article by F. J. Callaghan et al., "Relationship Between Pulse-Wave Velocity and Arterial Elasticity", Medical & Biological Engineering & Computing, May 1986, Pages 248 to 254.